Considerable amounts of secondary sludge are produced yearly in connection with biological waste-water treatment in paper and pulp mills. Secondary sludge is mainly organic material which can be separated from the mill waste-water as suspended matter. The disposal of secondary sludge is rather problematic, as all methods currently used are costly or difficult to employ. Conventionally secondary sludge has been disposed of by trucking it to a dumping ground or composting, or burning it together with an auxiliary fuel in a solid fuel boiler.
Before being trucked to a dumping ground, secondary sludge must be pretreated. Furthermore, the treated waste material requires a great deal of space. All this is rather costly. The secondary sludge must be composted with various additives, such as bark suspension or the like. The humus which is obtained by composting and which is in principle fit for use may be difficult to sell, and it is fairly expensive to dispose of it in some other way. If secondary sludge is burnt in a solid fuel boiler, it must be mixed with at least the same amount of bark or fibre suspension. The resulting mixed suspension has a low dry solids content, approximately 30%, and therefore the burning requires an auxiliary fuel. Such burning is financially unprofitable. Another factor that increases the costs is that the fibre suspension which would thus be required to be burnt with the secondary sludge could otherwise be recirculated to the process as fibrous raw material and sold with the final product. The burning of secondary sludge in a solid fuel boiler may also involve risks because so-called super toxins, such as dioxins, may be formed from chlorine compounds at low temperatures. It is thus not recommendable to dispose of secondary sludge in this way. Moreover, the burning of secondary sludge in a solid fuel boiler increases corrosion of the heat transfer surfaces on account of low fuel temperature and the chemicals contained in the secondary sludge.
The publication "Disposal of secondary sludge in the kraft recovery system" by W. J. Frederick, T. M. Grace and T. W. Joyce, Proc. Environmental Conf. Chem. Soc. Dir. Inst. Pap. Chem., Appleton, Wis., USA 1980, pp. 43-47, discloses the burning of secondary sludge in a recovery boiler. According to this publication, fairly small amounts of secondary sludge were added to black liquor before evaporation of the black liquor. In order for the fouling of heat transfer surfaces in the evaporation plant to be reduced, the secondary sludge was treated with white liquor before being added to the black liquor. In this publication it was found to be problematic that the separation of soap is less efficient when the black liquor comprises secondary sludge.
It is also known to burn secondary sludge by mixing it with black liquor as, for example, in the solution disclosed in Finnish Patent No. 80,664. This patent discloses a method in which soap separated from black liquor and acid are added to secondary sludge so as to obtain a mixture having a pH of 2 to 5. Thereafter the sludge is dewatered by pressing to obtain a dry solids content of about 20 to 25%. The sludge is then added to the black liquor prior to the evaporation plant. In the evaporation plant, soap is separated from the black liquor comprising sludge, and the soap is reused for the treatment of secondary sludge. This solution requires that the sludge be concentrated by pressing so that it would be suitable for the process. It further requires the use of additional acid, which increases the treatment costs. Even in this solution the separation of soap is less efficient because the liquor contains sludge, which further reduces the usefulness of the method. The fouling of heat transfer surfaces is also an obvious risk in the method according to Finnish Patent No. 80,664.